By Margaret Sullivan,

But when his son, Donald Trump Jr., was forced to come clean about a meeting last year with a Russian lawyer to gather dirt on Hillary Clinton, Trump Jr.’s emails seemed to prove the opposite: There was a there there, after all. Undeniably so.

Even some members of the pro-Trump media were stunned: “So, like, this is straight-up collusion. Right?” reacted a top Breitbart editor in an internal Slack message, according to CNN’s Oliver Darcy. (The editor later said his remark was taken out of context.)

Some in the mainstream media were quick to gloat that this would be the end of the denials. How could he claim the news media were lying when, for once, the proof was right there? And not from ghostly unnamed sources but with real names attached.

But a day later, the president was back to ranting about how journalistic sources are fabricated. (He tweeted: “Remember, when you hear the words ‘sources say’ from the Fake Media, often times those sources are made up and do not exist.”)

He had hardly skipped a beat.

“Yes, they are forced to admit that a meeting happened — but everything else is still ‘fake news,’ ” said Joel Kaplan, an associate dean at Syracuse University’s Newhouse School.

Consider the head-spinning words of White House spokeswoman Sarah Huckabee Sanders: “The only thing I see inappropriate about the meeting was the people that leaked the information on the meeting after it was voluntarily disclosed.” (That isn’t how it happened but hey, leaks are always fake news in Trump World.)

The fake-news defense will live on, Kaplan said, for one simple reason: It works.

“The mantra is not going to go away. It has resonance — it’s the replacement for the claim of liberal bias, and is a far more effective one.”

Of course, for Trump skeptics, the fake-news defense has never been anything but a diversion.

But for Trump’s base, and some independents, Kaplan said, “it does call the reliability and credibility” of the mainstream media into question.

And the media’s own flaws help to foster that doubt. When CNN fired three journalists after a story was retracted; when Newsday, the Long Island newspaper, announced a few days ago that it was unable to verify the existence of more than 100 sources of a reporter; and when people recall famous journalism scandals such as those involving Jayson Blair at the New York Times or Stephen Glass at the New Republic, the president’s fake-news attacks get a foothold.

For most journalists, these scandals prove not that the news media is full of liars and cheats, but that such incidents are so rare as to be a big deal when they happen. Mainstream news organizations, for the most part, take accuracy and accountability seriously enough to self-investigate and ’fess up.

But not everyone buys that.

“People come to news with their own built-in biases — we filter information that way,” said Nikki Usher, associate professor at George Washington University’s School of Media and Public Affairs.

So those who want to see collusion will agree with Mother Jones’s David Corn: “The Trump campaign shielded a foreign adversary that sought to destabilize American democracy.”

Others will shrug it off.

“There’s been so much negative coverage of this administration — arguably, rightfully so — that it’s hard for most people to know what counts,” Usher said. “News junkies are obsessed with it, but a lot of others are going to tune it out.”

Or blame the messenger.

One conservative radio host, Robert Leonard, told CNN’s Brian Stelter that his conservative friends in Iowa “are very angry — they think the Trump/Russia deal is a coup attempt by the media. They don’t understand why the media is trying to oust our duly elected president.”

Those reactions shouldn’t diminish the value of the journalistic digging — in this case by the New York Times — that forced Trump Jr.’s hand. He tweeted out his emails just before the Times was due to publish them.

The Trump camp’s virtuous claims of “transparency” were laughable, but only if you were following the story closely, Usher said. And how many were paying attention later in the week when NBC News reported that a Russian American lobbyist,with a Soviet military background, also attended the meeting?

Trump Jr.’s tweeting out his emails, Usher said, “was a brilliant move — a way of going directly to the audience, and seizing control of the story.”

Despite the bombshell, there was a speedy effort by the Trump administration to return to business as usual: Chaos, distraction, and information overload, accompanied by a presidential kiss-off — “Most people would have taken that meeting,” Trump said.

We’re in a golden era for accountability journalism, but that doesn’t mean everyone is going to hear what the watchdogs are barking about. Even if they do, they may not believe it.

And that, after all, is the aim of Trump’s fake-news defense. Which is exactly why it’s here to stay.